1. Field of the Invention
This invention relates to the field of age-hardenable aluminum alloys suitable for aerospace and other demanding applications. The invention further relates to now aluminum alloy products having improved combinations of strength and toughness suitable for high speed aircraft applications, especially fuselage skins and wing members. For such applications, resistance to creep and/or stress corrosion cracking may be critical.
2. Technology Review
One important means for enhancing the strength of aluminum alloys is by heat treatment. Three basic steps generally employed for the heat treatment of many aluminum alloys are: (1) solution heat treating; (2) quenching; and (3) aging. Some cold working may also be performed between quenching and aging. Solution heat treatment consists of soaking an alloy at a sufficiently high temperature and for a long enough time to achieve a near homogeneous solid solution of precipitate-forming elements within the alloy. The objective is to take into solid solution the most practical amount of soluble-hardening elements. Quenching, or rapid cooling of the solid solution formed during solution heat treatment, produces a supersaturated solid solution at room temperature. Aging then forms strengthening precipitates from this rapidly cooled, supersaturated solid solution. Such precipitates may form naturally at ambient temperatures or artificially using elevated temperature aging techniques. In natural aging, quenched alloy products are held at temperatures ranging from -20.degree. C. to +50.degree. C., but most typically at room temperature, for relatively long periods of time. For some alloy compositions, precipitation hardening from just natural aging produces materials with useful physical and mechanical properties. In artificial aging, a quenched alloy is held at temperatures typically ranging from 100.degree. to 190.degree. C. for time periods typically ranging from 5 to 48 hours, to cause some precipitation hardening in the final product.
The extent to which an aluminum alloy's strength can be enhanced by heat treatment varies with the type and amount of alloying constituents present. For example, adding copper to aluminum improves alloy strength and, in some instances, even enhances weldability to some point. The further addition of magnesium to such Al-Cu alloys can improve that alloy's resistance to corrosion, enhance its natural aging response (without prior cold working) and even increase its strength somewhat. At relatively low Mg levels, however, that alloy's weldability may decrease.
One commercially available alloy containing both copper and magnesium is 2024 aluminum (Aluminum Association designation). A representative composition within the range of 2024 is 4.4 wt. % Cu, 1.5 wt. % Mg, 0.6 wt. % Mn and a balance of aluminum, incidental elements and impurities. Alloy 2024 is widely used because of its high strength, good toughness, and good natural-aging response. In some tempers, it suffers from limited corrosion resistance, however.
Another commercial Al-Cu-Mg alloy is sold as 2519 aluminum (Aluminum Association designation). This alloy has a representative composition of 5.8 wt. % Cu, 0.2 wt. % Mg, 0.3 wt. % Mn, 0.2 wt. % Zr, 0.06 wt. % Ti, 0.05 wt. % V and a balance of aluminum, incidental elements and impurities. Alloy 2519 developed as an improvement to alloy 2219, is presently used for some military applications including armor plate.
According to U.S. Pat. No. 4,772,342, Polmear added silver to an Al-Cu-Mg-Mn-V system to increase the elevated temperature properties of that alloy. One representative embodiment from that patent has the composition 6.0 wt. % Cu, 0.5 wt. % Mg, 0.4 wt. % Ag, 0.5 wt. % Mn, 0.15 wt. % Zr, 0.10 wt. % V, 0.05 wt. % Si and a balance of aluminum. According to Polmear, the increase in strength which he observed was due to a plate-like .OMEGA. phase on the {111 } planes arising when both Mg and Ag are present. While the typical tensile yield strengths of Polmear's extruded rod sections measured up to 75 ksi, this inventor could not repeat such strength levels for other property forms. When sheet product was made using Polmear's preferred composition range for comparative purposes, such sheet product only exhibited typical tensile yield strengths of about 70 ksi compared to the 77 ksi or higher typical strength levels observed with sheet product equivalents of this invention.